1. Field of the Invention
The present invention relates to an optical recording medium, such as a so-called compact disk (CD) or the like and which is of such a system as to reproduce a signal by detecting a changed amount of a reflection of a reproduced light beam with radiating from a substrate side a reproducing light on recording patterns such as concavities and convexities corresponding to an information signal recorded on a transparent substrate or recorded pits recorded by the change of optical characteristics of the medium.
2. Description of the Related Art
Recently, in the field of information recording, researches concerning the optical information recording system have hitherto been advanced in various places. The optical information recording system has lots of advantages, such as to effect the recording and reproducing in a non-contact fashion, to achieve high recording density which is higher than that of the magnetic recording system by one figure, to be applicable to several memory types of read-only type, write-once type, rewritable type or the like. Therefore, this optical information recording system can realize an inexpensive large storage capacity file and a variety of uses from professional to consumer uses can be considered.
As the read-only type optical recording medium of the above-mentioned types, a digital audio disk (i.e., so-called compact disk, CD), an optical video disk (so-called laser disk, LD) or CD-ROM have already been widespread. In these optical recording media, a reflection layer made of a metal thin film such as an A1 film or the like is deposited on the surface of a substrate having a layer in which concavities and convexities corresponding to an information signal or record patterns based on the change of optical characteristics of the medium are formed. A reproducing light such as a laser light or the like is radiated on the optical recording medium from the transparent substrate side and information is reproduced by detecting intensity of a reflected light.
In the optical recording medium of the above-mentioned read-only type, there is a demand that the recording density thereof is further improved in order to cope with digital VTRs, high-definition television receivers (HDTVs) or the like. Further, there is a demand that optical recording media are miniaturized in size in order to facilitate the operation thereof. From this standpoint, a further improvement of the recording density is requested.
As means for improving the recording density of the optical recording medium, it is considered to provide smaller recording pattern corresponding to information formed on the optical recording medium. If the recording pattern were made very small, then a spot diameter of a reproducing light that reproduces the optical recording medium would exceed a diffraction limit and the recording pattern would not be reproduced.
Therefore, in order to reduce the wavelength of the reproducing light, to compress a signal code or to improve the above-mentioned diffraction limit, there are considered various methods, such as increase of a numerical aperture (NA) of the optical system or the like. Of these methods, a super resolution receives a remarkable attention as a powerful means.
A principle of super resolution is such that a spot diameter of the radiated light is reduced to be smaller than the diffraction limit by setting an aperture smaller than the diffraction limit of the radiated light at object point position to thereby increase a resolution (e.g., "Charles W. McCutchenm "Super-resolution in Microscopy and the Abbe Resolution Limit." Journal of Optical Society of America, 57(10), 1190 (1967)", "Tony Wilson and Colin Shepard, "Theory and Practice of Scanning Optical Microscopy." Academic Press (London), 1984, etc.).
If such super resolution is applied to the optical recording medium, then the aperture must be moved as the reproducing light is moved on the optical recording medium. U.S. Pat. No. 5,153,873 described a technology that an aperture smaller than the diffraction-limited radiation spot) is formed within the reflection layer and resolution is improved by reducing the spot diameter of a reflected light reflected by the reflection layer.
In this previously-proposed method, as the optical response material, there is enumerated a non-linear optical material whose optical characteristic is directly changed by the reproducing light or a phase change material whose optical characteristic is changed by heat generated when the reproducing light absorbs a light. However, specific proper materials are not illustrated in the above-mentioned U.S. Pat. No. 5,153,873 and cannot be realized in actual practice.
Further, the assignee of the present application has previously proposed a method (now U.S. Pat. No. 5,018,119) in which super resolution effect is realized by reducing an area of a magnetic Kerr effect of a magneto-optical recording and reproducing system to be smaller than the spot diameter of the reproduced light thermally to thereby achieve the high density recording.